The present invention relates to homogeneous charge compression ignition (HCCI) internal combustion engine having a plurality of cylinders. More specifically, the present invention relates to control for reducing variation of combustion states among the cylinders.
In a typical HCCI internal combustion engine, fuel and air is premixed before reaching cylinders. The mixture is compressed by pistons to self-ignite. This type of engine is similar to a diesel engine in that compression ignition is performed without spark plugs. On the other hand, the engine is similar to a gasoline engine in that fuel is premixed with air before being igniting. This type of engine thus has a quality between a diesel engine and a gasoline engine.
This type of internal combustion engine provides various advantages such as high heat efficiency, improved fuel consumption, reduction in the emission harmful substances such as soot and NOx. Research and development of this engine have been thus enhanced. One of major obstacles to the practical application of HCCI internal combustion engines is that control of ignition and combustion is difficult as compared to diesel engines and gasoline engines.
That is, in a diesel engine or a gasoline engine, it is possible to perform ignition at the most appropriate time about the piston top dead center by adjusting the timing to inject fuel into the cylinder or the timing to cause the spark plug to spark. On the other hand, in the case of an HCCI engine, fuel and air are mixed in advance, and the mixture is compressed so that it spontaneously self-ignite. Thus, control factors including the temperature, pressure, and fuel concentration must be appropriately controlled such that self-ignition occurs around the piston top dead center. This makes the control extremely difficult. Therefore, the engine can be operated only in a narrow range where such control is not difficult.
On this point, Japanese Laid-Open Patent Publication No. 2004-190539 points out the following. That is, in a multicylinder two-cycle engine, the efficiency of scavenging can vary from one cylinder to another due to pressure pulsation in the intake pipes and exhaust pipes. As a result, the combustion state varies from one cylinder to another. Therefore, application of HCCI to a multicylinder two-cycle engine substantially reduces the range of operating condition in which compression ignition can be performed.
The above publication discloses an improved engine to solve the problem. The disclosed engine includes a sensor for detecting combustion state information related to the combustion state in each combustion chamber. The engine also includes a computation section that, based on information detected by the sensor, computes a specific combustion state index representing the combustion state in each combustion chamber. The computation section also computes an average combustion state index, which is the mean value of the combustion state indexes of all the combustion chambers. Further, the engine includes a combustion control parameter adjusting section that determines whether the combustion state index in each combustion chamber is in a predetermined permissible range around the average combustion state index. When the combustion state index of a combustion chamber is out of the permissible range, the combustion control parameter adjusting section adjusts a combustion control parameter of the combustion chamber, such that the combustion states of all the combustion chambers approach one another.
Specifically, the combustion control parameter adjusting section increases or reduces the fuel injection for each combustion chamber, or advances or retards the closing timing of the intake valve of each combustion chamber, thereby causing the combustion states of all the combustion chambers to resemble one another.
Since the engine of the above publication operates with all the combustion chambers in similar combustion states, compression ignition is prevented from being inappropriate in some of the combustion chambers. As a result, the range of compression ignition operation of the entire engine is expanded.
However, changing of the fuel injection amount or valve closing timing of the intake valves requires a structure for changing the fuel injection amount for each cylinder or a mechanism for changing the valve closing timing of the intake valve for each cylinder. This complicates the structure and thus hinders reduction in the weight and simplification of the control.